Pulverizing mills are used to pulverize coal, limestone and other solid materials. In the case of coal, gravel sized coal enters the mill and is pulverized into a powder. The powder is carried out of the pulverizing mill by a high velocity air stream and into a furnace where it explosively burns to heat steam which, in an electrical power generator, drives a turbine to generate electricity. Pulverizing mills are designed to operate continuously, except during periods of repair. Examples of these kinds pulverizing mills are described in U.S. Pat. No. 4,705,223 by Dibowski et al.; U.S. Pat. No. 4,694,994 by Henne et al.; U.S. Pat. No. 4,679,739 by Hashimoto et al.; U.S. Pat. No. 4,522,343 by Williams; U.S. Pat. No. 4,491,280 by Bacharach; and U.S. Pat. No. 4,717,082 by Guido et al.
Pulverizing is accomplished by directing the coal onto grinding tables which interface with pulverizing rollers. The rollers are each mounted on a separate roller assembly shaft, and each roller assembly shaft is mounted on a clamshell door in the pulverizing mill. Typically, the grinding table is a disk-shaped structure with an annular groove or raised circumferential edge in the top surface. The grinding table rotates so that the annular groove mates with the rollers. The coal is introduced from the top of the assembly and feeds by gravity to the annular groove where it is pulverized as the grinding table rotates under the rollers. The pulverized coal dust is discharged from the grinding table by a high velocity air flow deflected over the grinding table. The coal dust is redirected through and out of the pulverizing mill by subsequent deflection of the combined flow of air and suspended coal dust particles.
Pulverizing mills may use a rotating grinding table with stationary roller assemblies, as described in U.S. Pat. No. 4,717,082 by Guido et al. (the contents of which are hereby incorporated by reference), and additional examples of these kinds of roller assemblies are described in U.S. Pat. No. 4,996,757 by Parham and U.S. Pat. No. 5,050,810 also by Parham. Alternatively, pulverizing mills may use a stationary grinding table and several rotating roller assemblies. The roller assemblies may also be independently biased against the grinding table so that vibration and shock on one roller will not be transferred to all the other rollers, as described in the Guido et al. '082 patent. The rollers and grinding table are massive. Each roller typically weighs several tons and is on the order of five feet in diameter.
The roller assemblies are biased towards the grinding table by means of compression spring assemblies. Because of the large size of typical pulverizing mills and grinding rollers, compression spring assemblies commonly exert forces within the range of 25,000 to 30,000 PSI. The compression spring assemblies typically are housed in a plunger can structure (sometimes referred to in the art also as a “Journal Spring Housing” or “Spring Housing” as a constituent part of a “Mechanical Spring System”) which is suitably mounted so as to cooperate with the roller assembly. A typical plunger can structure houses several elements, including a compression spring assembly, a plunger assembly which transfers the force generated by the compression spring to the roller element of the roller assembly, and a plunger bearing assembly, all of which are well known in the art (the plunger assembly is sometimes referred to in the art as a “Stud Assembly” or “Preload Stud Assembly”). Examples of these kinds of plunger can structures and the assemblies housed therein are described in U.S. Pat. No. 3,881,348 by Morton; U.S. Pat. No. 4,706,900 by Prairie, et al.; and U.S. Pat. No. 4,759,509 by Prairie.
The plunger can structure itself, as well as the compression spring assembly, the plunger assembly, the plunger bearing assembly, and all of the interfacing and other elements of each assembly contained within the plunger can, are exposed to extreme conditions. The massive grinding table with which the roller assemblies cooperate typically revolve at 200 to 300 revolutions per minute. The pulverizing mills within which many of the plunger cans are installed operate at temperatures between 600 to 700 degrees Fahrenheit. In addition, the mills occasionally catch fire. Such fires are frequently smothered with steam and then cooled, resulting in large and fast temperature changes within the pulverizing mills. There is also the constant presence of pulverized coal dust particles throughout the pulverizing mills. Carried by high speed air flow, the coal particles in motion create the effect of a continuous sand-blasting on all component structures within the interior of the pulverizing mill.
The existing multi-part fabricated plunger can structure, cooperating with its several multi-part assemblies and interfacing elements under the extreme conditions of the pulverizing mill, is a source of a number of costly problems. These problems affect both the fabricated plunger can structure and the assemblies it houses. One problem is that the fabricated plunger can wears out or one or more of the multiplicity of parts comprising it wears out. Such wear in the fabricated plunger can is a product of vibration, abrasion, and shock, and is accentuated by differential shrinkage and expansion of its various elements in reaction to heating and cooling within the pulverizing mill. Stress cracks and fractures are not uncommon in the fabricated plunger can structure. So also, and by similar causes, the compression spring assembly, plunger assembly, plunger bearing assembly and interfacing elements contained within the fabricated plunger can structure experience structural degradation, deterioration, misalignment and wear. Other degradation to the assemblies is caused by the cumulative blasting effect and deposit over time, and consequent caking of, coal dust particles around the elements of such assemblies.
Repairing the existing fabricated plunger can structures themselves, and opening them so as to inspect, clean, adjust, or repair or replace the compression spring assembly, plunger assembly, plunger bearing assembly, and interfacing elements contained within them presents other difficulties. The compression spring in the plunger can typically is under twenty to thirty thousand pounds or more of pressure. The top tends to explode off the can like a bomb when it is removed, thereby endangering any workmen in the immediate surroundings. Also, the existing fabricated plunger can structures must be removed from the pulverizing mill for opening off site. This requires extensive labor and consumes valuable time. The pulverizing mill cannot operate during that time, and the down time imposes a cost of many thousands of dollars per day. Electric utilities must either pass that cost on to rate payers or else absorb it so as to suffer diminished rates of return to their shareholders. An improved plunger can assembly addressing these concerns is described in U.S. Pat. No. 5,242,123 by Parham.
Moreover, wear and degradation to the plunger can structure and to the assemblies housed within it adversely affect the massive roller assemblies of the pulverizing mill. In particular, the wear rate of the roller assemblies is sensitive, not only to the depth, hardness and uniform size and consistency of the coal, but also to the amount and uniformity of the countervailing force applied to the rollers by the compression spring and other assemblies housed within the plunger can structure. The cost of repairing or replacing the rollers is very high in relation to the cost of repairing or replacing the plunger can structures and any of the assemblies contained therein.
One particularly formidable problem presented by plunger can structures relates to the interface between the plunger can and the roller assemblies. In some pulverizing mills, the plunger tip rides on the roller assembly to provide a biasing force urging the roller assembly down onto the grinding table to grind the coal. As the rollers wear, however, more play is introduced into the system as the compression spring expands in response to the roller wear. Because the force exerted by the compression spring against the plunger tip, and consequently by the plunger tip against the roller assembly, is proportional to the spring compression, this expansion of the compression spring reduces this force. As the roller wear continues, the force reduction, which results in less pulverizing effect, becomes unacceptable. At that point, it is necessary to place a shim between the plunger tip and the roller assembly to take up the play resulting from the roller wear to bring the force exerted by the plunger tip on the roller assembly back up to the desired level.
This shimming operation is time consuming, which results in high labor costs and expensive mill down-time. It is generally necessary to open the clamshell doors to the mill on which the plunger assembly is mounted, apply the necessary shims, and then close the clamshell doors. The opening and closing of the clamshell doors is an elaborate and dangerous procedure.